Recently, a light-emitting element using a material that emits light (electroluminescence) by applying an electric field (hereinafter, referred to as “EL material”) has been actively researched as a self light-emitting element. In particular, an element using an organic compound as the EL material (hereinafter, referred to as “organic light-emitting element”) has been attracting attention. Further, various light-emitting devices using the organic light-emitting element has been proposed, which is expected to be applied to displays.
An organic light-emitting element has a structure in which a layer including an electroluminescent organic compound (hereinafter, referred to as “organic light-emitting layer”) is interposed between an anode and a cathode, and has an emission mechanism in which, by applying a voltage between electrodes, an electron injected from a cathode and a hole injected from an anode are recombined in the electroluminescent layer to form a molecule in an excited state (hereinafter, referred to as an excited molecule), and energy is released to emit light while the excited molecule moves back toward the ground state. The foregoing light emission from the excited molecule is referred to as luminescence.
Luminescence in an organic compound includes luminescence during moving back to a ground state from a single excited state (fluorescence) and luminescence during moving back to a ground state from a triplet excited state (phosphorescence), and the specification includes a case of using either luminescence.
In the organic light-emitting element, the organic light-emitting layer generally has a laminated structure. Typically, a laminated structure of “a hole transport layer/a light-emitting layer/an electron transport layer” can be given (refer to Non-Patent Document 1). This structure has a quite high luminous efficiency, and organic light-emitting elements for which research and development has been advanced presently are mostly constructed based on this structure.
(Non-Patent Document 1)
C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)
In addition, a structure in which ‘a hole injection layer/a hole transport layer/a light-emitting layer/an electron transport layer’ or ‘a hole injection layer/a hole transport layer/a light-emitting layer/an electron transport layer/an electron injection layer’ are in sequence laminated on anode is suggested. Further, a method such as doping a light-emitting layer with a fluorescent pigment is also proposed. Besides, materials that are used for these layers include a low molecular weight material and a high molecular weight material (polymer), and the low molecular weight material is deposited mainly by vacuum deposition while a high molecular weight material is deposited by a wet coating method.
Since all layers provided between a cathode and an anode are collectively called an organic light-emitting layer in the specification, the above-mentioned hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer are all included in the organic light-emitting layer.
In the case of applying the organic light-emitting element as described above to a light-emitting device, particularly, to a use for displaying such as a display, it is a feature that there is no problem with a viewing angle unlike a liquid crystal display device. In other words, the light-emitting device is more appropriate as a display for outdoor use than a liquid crystal display, and uses in various forms are proposed.
The mode of a display includes a mode in which an organic light-emitting layer is formed between two-kinds of stripe-shaped electrodes provided to be mutually perpendicular (a simple matrix mode) and a mode in which an organic light-emitting layer is formed between a pixel electrode connected to a TFT (thin film transistor) and arranged in a matrix shape and an opposed electrode (an active matrix mode), in addition to a segment. In the case of a pixel density increased, the active matrix mode that has a switch provided with respect to each pixel (or 1 dot) has an advantage since driving at a lower driving voltage is possible as compared to the simple matrix mode.
By the way, in the above-mentioned organic light-emitting element, a material composing the organic light-emitting layer easily oxidizes or absorbs moisture due to an existence of oxygen or water to degrade, which results in a problem that the luminance of the organic light-emitting element lowers with time.
There, an arrival of oxygen or an arrival of moisture at an organic light-emitting element is prevented conventionally by a method of providing a protective case around an organic light-emitting element and filling fine powder of a solid dehydrating agent in the case (refer to Patent Document 1) or a method of encapsulating an organic light-emitting element in an airtight container formed of a sealing can or the like and attaching a desiccant to be away from the element (Patent Document 2).
(Patent Document 1)
Japanese Patent Laid-Open 6-176867
(Patent Document 2)
Japanese Patent Laid-Open 9-148066